The efficiency of Cu(In,Ga)Se2 (CIGS)-based solar cells can be markedly improved by controlled introduction of alkali metal (AM) atoms using post-deposition treatment (PDT) after CIGS growth. Previous studies have indicated that AM atoms may act as impurities or agglomerate into secondary phases. To enable further progress, understanding of atomic level processes responsible for these improvements is required. To this end, we have investigated theoretically the effects of the AM elements Li, Na, K, Rb, and Cs on the properties of the parent material CuInSe2. First, the effects of the AM impurities in CuInSe2 have been investigated in terms of formation energies, charge transition levels, and migration energy barriers. We found that AM atoms preferentially substitute for Cu atoms at the neutral charge state. Under In-poor conditions, AM atoms at the In site also show low formation energies and are acceptors. The migration energy barriers show that the interstitial diffusion mechanism may be relevant only for Li, Na, and K, whereas all the AM atoms can diffuse with the help of Cu vacancies. The competition between these two mechanisms strongly depends on the concentration of Cu vacancies. We also discuss how AM atoms can contribute to increasing Cu-depleted regions. Second, AM atoms can form secondary phases with Se and In atoms. We suggest a mechanism for the secondary phase formation following the PDT process. On the basis of the calculated reaction enthalpies and migration considerations, we find that mixed phases are more likely in the case of LiInSe2 and NaInSe2, whereas formation of secondary phases is expected for KInSe2, RbInSe2, and CsInSe2. We discuss our findings in the light of experimental results obtained for AM treatments. The secondary phases have large energy band gaps and improve the morphology of the buffer surface by enabling a favorable band alignment, which can improve the electrical properties of the device. Moreover, they can also passivate the surface by forming a diffusion barrier. Overall, our work points to different roles played by the light and heavy AM atoms and suggests that both types may be needed to maximize their benefits on the solar cell performance.